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First Posted: Apr 5, 2011
Last Updated:Aug 1, 2013

Molds, Mycotoxins and the Horse

What are mycotoxins? What effect do they have on horses? Are they dangerous? Can they be prevented? Is there a treatment for a horse effected by mycotoxins? The following article is from the Ministry of Agriculture, Food and Rural Affairs, Ontario, Canada. It is written by Dr. Bob Wright - Veterinary Scientist/OMAFRA. To read the full article follow the provided link. Molds, Mycotoxins and their Effect on Horses

Thank you for official license from your Copyright Unit (Ministry of Agriculture, Food and Rural Affairs, Ontario, Canada) to reprint the following quoted passages from Dr. Bob Wright's article on "Molds, Mycotoxins and their Effect on Horses."

Molds, Mycotoxins and their Effect on Horses
© Queen's Printer for Ontario, 2003. Reproduced with permission
Dr. Bob Wright/Creation Date: 01 September 2003/Last Reviewed:01 January 2005

"Pasture grasses, hay, grain, straw and stubble can all support the growth of various fungi. The fungi can exist as saprophytes, living on the outside of the plant and obtaining nutrients from the plant with no benefit to the plant, or exist as endophytes within the plant in a symbiotic relationship, providing benefits to the plant while obtaining nutrients from the plant. The saprophytes include the more common genera Aspergillus, Claviceps, Stachybotrys, Fusarium and Penicillium. The endophytes live between the plant's cell walls and include the more common genera Balansia, Epichloe, Acremonium and Neotyphodium.

Fungi and their associated mycotoxins are present on grain crops in varying amounts each year, depending on the climatic growing conditions. A cool, wet, growing season increases the likelihood that fungi, especially Fusarium and its mycotoxins, will be present in small grains. The high moisture level in grain encourages fungal growth while the cool temperatures increase the production of mycotoxins. Mycotoxins are secondary metabolites produced by fungi and can affect various animals upon ingestion or inhalation. High concentrations of mycotoxins in wheat grain are unusual. Normally the concern is with wet falls and the invasion of corn and corn byproducts by fungi. The research involving horses and mycotoxins is limited when compared to research in other species, such as cattle and swine.

The reporting of mycotoxin concentrations may be expressed in a number of ways. Therefore, remember;
1 ppm = 1 mg/kg = 1Micro-gram/g Example: aflatoxin @ .2 ppm is 200 ppb
1 ppm = 1,000 ppb DON @ 1200 ppb is 1.2 ppm"

Note: In science and engineering, the parts-per notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g, mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they are pure numbers with no associated units of measurement. Commonly used are ppm (parts-per-million, 10-6), ppb (parts-per-billion, 10-9), ppt (parts-per-trillion, 10-12) and ppq (parts-per-quadrillion, 10-15).

Note: In science an even smaller concentration a ppb is a measurement: It denotes parts per billion (ppb). One ppb is one part in 1 billion.


The common mycotoxin-producing saprophytic fungi are Aspergillus, Claviceps, Stachybotrys, Fusarium and Penicillium. Limited or no information is available for the effects of Aspergillus, Stachybotrys and Penicillium and their associated mycotoxins on horses. ...

Mycotoxin Levels for Horses

The following table provides some guidelines for the tolerance levels of mycotoxins in the total ration dry matter (TRDM) above which they are potentially harmful.

Prevention and Recommendations

Research into the effects of mycotoxins on horses is in its infancy. As a general rule, err on the side of caution.

  • Feed and bed horses with hay/grain and bedding free of mycotoxins.
  • The amount of DON in cereal straw depends primarily on the presence of contaminated grain and chaff. Tedding or raking prior to baling the straw will reduce the chaff and kernels and, therefore, the concentrate of mycotoxins.
  • Since the potential concern with horses is the risk of an additive effect, in harvest years with high DON concentrations, consider limiting the use of straw bedding; ensure horses are fed high quality hay and in sufficient amounts to limit the eating of bedding.
  • Late-gestation mares should never be allowed to consume cereal-rye straw, endophyte-infected hay or stubble or eat sclerotia-containing grain. All pasture/hay seed mixes containing tall fescue and perennial ryegrass must be guaranteed endophyte-free.
  • Mow grass pastures to keep them in a vegetative state rather than letting them develop seed heads. This will minimize the opportunity for proliferation of airborne ascospore and the invasion of pasture plants.
  • Check with your laboratory. HPLC methodology may be more sensitive than thin layer chromatography (TLC) when trying to identify ergot alkaloid concentrations less than 1 ppm.
  • Mares can be treated with domperidone should an ergot alkaloid problem be suspected.

Binding Agents

Feed manufacturers often utilize binders of various types in feeds to tie up mycotoxins. These include clay-based binders and yeast cell wall extracts (e.g., Bio-Mos-M). Raymond et al. concluded that supplementation with 0.2% yeast cell wall extract (a polymeric glucomannan mycotoxin adsorbent) improved feed intake over feeding contaminated feed alone but not when compared to control diets not containing mycotoxins (2). The long-term effects of binding agents are not clear; therefore, they should be used with caution."

Reference (2) above: Raymond SL, Smith TK, Swamy HVLN. Effects of feeding of grains naturally contaminated with Fusarium mycotoxins on feed intake, serum chemistry, and hematology of horses, and the efficacy of a polymeric glucomannan mycotoxin adsorbent. J. Anim. Sci. 2003; 81:2123-2130.

Common Mycotoxins

Fumonisin (Fusarium spp.) - Fumonsins do well in moldy corn and can cause poisoning. Sometimes this poisoning is referred to as blind staggers. Lameness and/or staggering and seizures can occur if a horse eats more than 5 parts per million. Fumonsin toxicity is usually fatal. Central nervous system disease and death can occur a day or so after the horse has ingested the toxin. It is extremely important to have clean corn. It has also been suggested to feed only limited amounts of corn to horses.

Aflatoxins - Fever, sluggishness, diarrhea, blood in the stool, off food, liver and kidney damage, tumors, immune function and birth defects can be caused by the ingestion of aflatoxins at a rate greater than 50 parts per billion. They are found in corn, milo and peanuts. Temperature and humidity affect the growth of aflatoxins. Insect damage and drought are also culprits in its growth. Foals are the most susceptible to the toxicity.

DON (deoxynivalenol or vomitoxin) - Temperature changes contribute to DON in corn and wheat. The concern level for DON in wheat for livestock is 4 parts per million (Wood, G.E., 1992). Horses will refuse to eat their food and lose weight. They will also have compromised immune systems and may have elevated liver enzymes: glutamate dehydrogenose (GDH) and gamma glutamyl transferase (GGT). DON is most prevalent in Canada and the central US. Straw can also be a culprit when used as bedding if found in concentrations from 0.5-2.7 parts per million.

Ryegrass tremorgens - Ryegrass tremorgens is caused by a tiny fungi called endophytes. If in jested by a horse it can cause ryegrass slobbers. The toxins affect part of the brain's cerebellum. Ataxia, staggering, trembling and head shaking are seen in horses as well as erratic behavior and collapse. Only the most severe cases cause death. Remove the horse from the toxic feed and the horse usually shows improvement within a week or two. A myotoxin binder will shorten recovery time.

Slaframine - This grows on red clover and other legumes. It prefers cool conditions and lots of moisture. Visible bronze colored or black spots or rings appear on the clover. A lab test can be done to identify its presence.

Red clover

Slaframine can cause excessive salivation referred to as 'slobbers.' Increased tear production, increased urination, bloating with associated colic, diarrhea, feed refusal, or abortion are also symptoms. The toxin is found in baled hay. It degrades over time. Remove your horse from the contaminated feed and improvement will begin immediately.


Endophyte Fescue Toxicosis
This can cause reproductive problems: lowered sperm count, increased gestation time, embryonic death, stillbirths, weak foals, lactation problems, and other reproductive problems. You will hear that pregnant mares should not pasture on fescue. Caused by an infection of the seed with an endophyte. Because fescue is generally a hearty plant, it is a favored pasture grass, particularly in the eastern United States.

Symptoms of a horse with endophyte fescue toxicity might present with these symptoms: weight gain, a rough hair coat, higher than normal body temperature, and decreased performance. Fescue endophyte toxins (ergot alkaloids)
can cause necrosis of the tail or ears. Blood vessels can constrict and decrease blood flow.

There is variation in how horses respond to mycotoxins. Each horse is unique as is their susceptibility. Two horses in the same pasture ingesting the same amount of mycotoxins can exhibit different symptoms ranging from severe to none at all. As always, if you suspect any suspicious behavioral changes or symptoms in your horse that cause you concern, call your vet.

For More Information:

Molds, Mycotoxins and their Effect on Horses
Molds and Mycotoxins In Horse Feed: Basic Facts/by: H.V.L.N. Swamy, DVM, PhD November 11 2010
Mycotoxin Feed Contamination Poses Health Risks for Horses
Damage From Within
Ryegrass Staggers: the Danger to Horses

Feeding Horses
Drought Damage Dangers: Nitrates and Mycotoxins
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